This invention relates to a centrifugal clutch having an engine overspeed relief valve and, more particularly, to a clutch and relief valve which permit relatively quick disengagement of the clutch when engine overspeed occurs and modulation of the clutch pressure during clutch engagement, and which prevents engine stall.
In vehicles, hydraulically operated clutches are frequently used in which pairs of clutch plates constituting a clutch pack are frictionally engaged through the use of fluid pressure to apply torque from an engine output shaft to a transmission input shaft. One type of friction clutches is known as a centrifugal clutch in which a hydraulic fluid chamber is provided by two, spaced-apart annular members which rotate with a circular flywheel coupled to the engine. One of the annular members constitutes a piston which is slidable with respect to the other annular member to apply pressure on the clutch pack to engage the clutch or to disengage the clutch when the pressure is released. Hydraulic fluid is introduced to the fluid chamber and, as the two members rotate, a rotating torus of fluid formed by centrifugal force creates an axial force acting against the piston to slide the piston for clutch engagement and disengagement purposes.
The centrifugal clutch also has an overspeed relief valve which is carried by, for example, the piston. The relief valve is also responsive to centrifugal force to protect the engine from damage due to engine overspeed. When overspeed occurs, the relief valve opens to discharge the hydraulic fluid in the fluid chamber, thereby releasing the frictional contact of the clutch pack plates and disengaging the clutch. When the engine is below overspeed, the relief valve closes to allow the fluid chamber to fill with fluid whereby the piston slides to engage the clutch through frictional contact of the pack plates.
While the prior centrifugal clutch having the relief valve performs satisfactorily, it does not necessarily provide as quick a response to engine overspeed as would be desirable for purposes of preventing engine damage. This is due in part to the time required for the relief valve to open when engine overspeed occurs and the time required to drain the fluid chamber once the relief valve is opened. Also, in such a prior centrifugal clutch, the flow rate of hydraulic fluid into the fluid chamber during clutch engagement is substantially constant. Since the flow rate at which the chamber is filled also determines the rate at which torque is applied from the engine output shaft to the transmission input shaft, there is substantially no control over the modulation of clutch pressure during engagement of the clutch.